Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib

Question

When starting up my web site for the first time, I\'m getting this error

Could not load type \'System.Runtime.CompilerServices.ExtensionAttribute\' from assemb

Answer 1

In my case I had an issue around using Microsoft.ReportViewer.WebForms. I removed validate=true from add verb line in web.config and it started working: